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Law of conservation of energy problems

  1. Oct 15, 2004 #1
    The problem:
    A mass m is attached to the end of a spring (constant k). The mass is given an initial displacement Xo from equilibrium, and an initial speed Vi. Ignoring friction and the mass of the spring, use energy methods to find (a) its maximum speed, and (b) its maximum stretch from equilibrium, in terms of the given quantities

    The answer I got for part a:
    [tex]v_{f} = \left(\frac{(kx_{0}^2 + mv_{i}^2)}{m}\right)^{1/2} = \left(\frac{kx_{o}^2}{m}\right)^{1/2} + v_{i}[/tex]

    - I set [tex]PE_{i} + KE_{i} = PE_{f} + KE_{f}[/tex] and crossed out [tex]PE_{f}[/tex] because max velocity occurs at zero potential

    The answer I got for part b:
    [tex]x = \left({\frac{mv_{i}^2}{k}}\right)^{1/2} = v_{i}\left(\frac{m}{k}\right)^{1/2}[/tex]

    - Again I used law of conservation of energy equation, and crossed [tex]KE_{f}[/tex] and [tex]PE_{i}[/tex] because the max stretch occurrs when [tex]PE_{f}[/tex] is maximum, therefore [tex]KE_{f} = 0[/tex]. The box's [tex]PE_{i} = 0[/tex] ( starting off from part a )

    What I want to know is whether these answers are correct, as I have absolutely no other way, except for my own intillect, to find out.
    Last edited: Oct 15, 2004
  2. jcsd
  3. Oct 16, 2004 #2

    Doc Al

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    Staff: Mentor

    This part is correct:
    [tex]v_{f} = \left(\frac{(kx_{0}^2 + mv_{i}^2)}{m}\right)^{1/2}[/tex]
    But that does not equal
    [tex]\left(\frac{kx_{o}^2}{m}\right)^{1/2} + v_{i}[/tex]
    Realize that [itex](a^2 + b^2)^{1/2} \ne a + b[/itex].

    The method is correct.

    This is incorrect. Your answer for b should be very similar to the answer for a, except that you solve for maximum X, instead of maximum V.
  4. Oct 16, 2004 #3
    silly mistake. Thanks.

    So I redid the second part and got this:
    [tex]\left(\frac{kx_{0}^2 + mv_{i}^2}{k}\right)^{1/2}[/tex]
  5. Oct 16, 2004 #4

    Doc Al

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    Staff: Mentor

    Looks good to me.
  6. Oct 16, 2004 #5
    thanks for the help
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